Many organic materials have been used in electronic devices. The use of organic materials for the fabrication of electronic devices such as field-effect transistors (FETs) has been studied in great detail and many organic substances that display interesting electronic properties have been synthesized. These materials provide low-cost integrated circuit (IC) technology suitable for applications such as smart cards, electronic tags, displays, and memory devices.
U.S. Pat. No. 6,452,207 discloses a class of fluorene oligomer compounds and describes a thin film transistor device that comprises a semiconductor layer of fluorene oligomer compounds. These fluorene oligomers are deposited by simple evaporation to achieve desirable semiconductor properties.
Other organic compounds that have been investigated for use as semiconductors include regioregular poly(3-alkylthiophene)s, oligothiophene derivatives, and fused aromatic compounds such as pentacene and tetracene. These compounds have also been found to exhibit semiconductor properties.
However, most of the above organic semiconductors compounds are p-channel. They possess a relatively high HOMO (highest occupied molecular orbital) energy level and are easily oxidized. This results in poor device stability and makes these compounds unsuitable for practical electronic circuit applications. Moreover, these compounds often exhibit high oxygen and moisture sensitivity, leading to poor on/off current ratios and limited environmental stability.
Furthermore, the synthesis of many of these compounds is low yielding. This leads to problems in purification of these compounds. Impurities present in these compounds, in turn, can lead to difficulties in achieving long range molecular ordering. This adversely affects the semiconductor properties such as field effect mobility and on/off current ratio.
Some phenylene-thiophene compounds, when used in electronic devices, are highly stable and remain unaffected by heat, air, or light. They also provide high field-effect mobility and high on/off ratio. A method for the preparation of some phenylene-thiophene compounds has been disclosed by S. R. Bayly, et al., Journal of the Chemical Society, Dalton Transactions, 2001, Volume 9, pp. 1401-1414. This publication describes electronic and magnetic metal-metal interactions in dinuclear oxomolybdenum (V) complexes and the dependence of these interactions on the nature of bridging ligands. The bridging ligands mentioned in this publication include some phenylene-thiophene compounds. However, the publication does not mention any other use of these phenylene-thiophene compounds, for instance, in electronic devices.
Despite the continuing interest in using organic semiconductors in electronic devices, there still exists a need for a class of organic compounds that have high mobility and high on/off ratio, and are stable to heat, light, and air. Furthermore, there is a need for organic compounds that can be readily prepared in high yield, easily purified, and incorporated into electronic devices using commercially viable fabrication methods.